Sunday, March 27, 2011
Primitive Body Plans
In chapter 6, Shubin talks about body plans. He talks about how humans have front/back, top/bottom, and left/right. Humans, and other recent animals, have bodies that are symmetrical. However, Shubin also talks about animals like jellyfish. Their bodies lack a front and back, a head and tail, and a left and right. Why is it that primitive animals like the jellyfish have such different body plans than more recent animals like us humans? What other animals are like the jellyfish and have body plans that are difficult to compare with our basic design? Danielle Webb (dwebb456@gmail.com)
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Jellyfish utilize a different body plan than terrestrial animals like humans because their environments require a completely different set of skills than we do. Their bodies lack distinct back/front portions because they exhibit radial symmetry; this is a selective advantage for them, because it allows jellyfish to stay highly mobile in currents or calm marine waters while easily detecting food or danger from any direction. In addition, jellyfish tentacles can be considered "tail" parts because they also primarily assist in mobility. While all jellyfish are radially symmetrical, their body plans have three layers of dermis (the outer epidermis, middle mesoglea and inner gastrodermis) to protect the mouth and gonads that grow near the center of its body. These qualities probably arose over time through periods of evolution, because the jellyfish that exhibited heightened mobility and protection of its reproductive/digestive parts had a higher chance of surviving natural selection. Moreover, the head/tail differentiation does not apply to jellyfsh because no cephalization has developed in jellyfish. Since we as humans require legs for mobility and do not live in marine environments, we don't necessarily need the same body plans or qualities that jellyfish have; note that most animals exhibiting radial symmetry, like sea stars, do not live in terrestrial environments.
ReplyDeleteSome other animals that have significantly different body plans than the human bilateral plan are sea stars, sea urchins, sea lilies and sea anemones that are all radially symmetrical, marine organisms like the jellyfish. However, while jellyfish are either floating or mobile in activity, the other radially symmetrical organisms are mostly sessile and thrive in one stable place on the sea floor.
Sources:
1.http://www.dnr.sc.gov/marine/pub/seascience/jellyfi.html
2. http://en.wikipedia.org/wiki/Symmetry_in_biology
Christine Lin
choco_cat11@comcast.net
In addition to what Christine said, another reason that primitive animals have different body plans than we do is because of the number of Hox genes that they have. This can be attributed to the fact that Hox genes are influential in controlling an organism's body plan. For example, sponges only have one Hox gene, but some insects have eight (Erwin, D). While thinking about evolution, this makes perfect sense. Insects have much more complex body plans than sponges, so they should have more Hox genes. Also, mammals, which are more complex than both insects and sponges, have 38 Hox genes (Erwin, D). These numbers make sense; throughout evolution, the number of Hox genes and complexity of organism body plans has increased together.
ReplyDeleteIn addition to what Christine said, it is hard to compare our body plans to animals with more primitive body plans because we have little in common with animals such as jelly fish. Cnidarians, including corals, box jellies, and medusae which all have bilateral symmetry are difficult to compare to our body plans (Berkeley). This is because not only do they have bilateral symmetry (as opposed to our radial symmtery), but they also lack body cavities and other advanced features that have evolved more recently.
Sources:
http://www.americanscientist.org/issues/feature/the-origin-of-animal-body-plans/5
http://www.ucmp.berkeley.edu/cnidaria/cnidaria.html
Marissa Lobl (marissa.lobl@gmail.com)
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ReplyDeleteLike Christine said, jellyfish have different body plans and exist in different environments than humans. Unlike humans, jellyfish are diploblastic, radially symmetrical, and have a gastrovascular cavity. For jellyfish, radial symmetry is selectively advantage because they can move in all directions (for those that are not sessile) and receive stimuli from all different directions. Tentacles surrounding the radial body plans of cnideria allow the cnidaria to capture prey by stinging prey, puncturing them and injecting poison. At the center of both polyp and medusa forms of cnidarians is the mouth/anus that leads to the gastrovasucular cavity in which the single mouth/anus opening is the channel to where the cnideria eats, digests food, and poops it out. As Christine said, cephalization does not exist in jellyfish/cnideria. Instead, there is a nerve net that exists throughout the cnidaria (Campbell 1065). The nerve net controls the contraction and expansion of the gastrovascular cavity (1065). Cnideria are a lot less complex than humans in this manner, but their body plans are utilized to survive. Also as Christine mentioned, humans do not live in water, and we need legs for mobility (Christine). Cnidaria can be both sessile (polyp form) and mobile (medusa form). According to Campbell, cnideria move “freely in the water by a combination of passive drifting and contractions of its bell-shaped body” (671). I do not see where tentacles fit into mobility of jellyfish as Christine said, but it is possible. Regardless, the contractions of the bell-shaped body is significant to this question because the radical symmetry of cnideria allow then to contract in all directions to respond to stimuli, escape predators, attack prey, ect. As for the polyps, they may be sessile, but their radial symmetry and nerve net allows them to sense prey in all directions and as Campbell says, the tentacles are “waiting for prey” (671).
ReplyDeleteFor clarity, in contrast to Christine’s comment about sea star’s being radially symmetrical, we learned in class that we would NOT consider sea star’s to be radially symmetrical rather we would consider them bilaterally symmetrical. If you looked at a sea star from its ventral or dorsal view, and placed a ruler on top of it, and then move the ruler in a circular direction, you would find that sea stars are not radially symmetrical. The star-shape is not like the garden pot diagram in Campbell. If you were not perfectly “cutting” the sea star down the middle of one of its rays, there would be no symmetry. Even though the madreporite in the sea stars technically makes them not bilaterally symmetrical because it’s “off-centered,” for class we consider them BILATERALLY SYMMETRICAL.
To expand upon Christine’s other examples, sponges have different body plans than us humans. Sponges lack true tissues, and have choanocytes including collar cells that are flagellated cells that ingest bacteria and tiny food particles. These sponges are mainly sessile unlike the medusa form of cnideria. What is interesting about sponges, which lack true tissues, is that they have no specific symmetry (i.e. they to not express bilateral or radial symmetry). However, this body plan or lack of one works for sponges because they have numerous amount of pores surrounding their bodies to allow water to enter the epidermis in the hopes of bring in food for the sponge. Within the pores lies the spongocoel, which is the cavity the food and water enter. The beating flagella of the choanocytes creates a current that draws water in through the pores and out through the osculum. The food particles are trapped in the mucus of the choanocyte and engulfed via phagocytosis to be digested or transferred to other cells (670).
Finally, as Marissa said, Hox genes are influential in body development. According to Shubin, “Versions of the Hox genes appear in every animal with a body” (110). Obviously though, they are not the same in every animal because then all animals would look alike. Through evolution, there have been mutations and changes in the sequences of Hox genes and the number of Hox genes in an organism, creating differences among animals. So for a less evolved animal such as a sea anemone, their Hox genes only play a role in the development of a front end and a back end of their bodies, where as in humans, Hox genes play a role in the development of the front end, back end, and central region of the human body.
ReplyDelete(Bobby Muttilainen, rmuttilainen@gmail.com)